Imaging of objects based upon the polarization or depolarization of light

ABSTRACT

A method and apparatus for imaging objects based upon the polarization or depolarization of light. According to one embodiment, there is provided a method for imaging the surface of a turbid medium, the method comprising the steps of: (a) illuminating the surface of the turbid medium with light, whereby light is backscattered from the illuminated surface of the turbid medium; (b) detecting a pair of complementary polarization components of the backscattered light; and (c) forming an image of the illuminated surface using the pair of complementary polarization components. Preferably, the illuminating light is polarized (e.g., linearly polarized, circularly polarized, elliptically polarized). Where, for example, the illuminating light is linearly polarized, the pair of complementary polarization components are preferably the parallel and perpendicular components to the polarized illuminating light, and the image may be formed by subtracting the perpendicular component from the parallel component, by taking a ratio of the parallel and perpendicular components or by using some combination of a ratio and difference of the parallel and perpendicular components.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of presently U.S.patent application Ser. No. 08/704,841, filed Aug. 28, 1996 now U.S.Pat. No. 5,847,394, which in turn is a continuation-in-part of presentlyU.S. patent application Ser. No. 08/573,939 now U.S. Pat. No. 5,719,399, filed Dec. 18, 1995, both of which are herein incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates generally to techniques for imagingobjects located in or behind turbid media and more particularly to anovel technique for imaging objects located in, at the surface of orbehind turbid media and, additionally, to a novel technique fordetecting ice, snow and the like on airplane wings and similar surfaces.

As can readily be appreciated, there are many situations in which thedetection of an object present in a turbid, i.e., highly scattering,medium is highly desirable. For instance, the detection of a tumorembedded within a tissue is one such example. One common technique fordetecting tumors in tissues uses X-ray radiation. Although X-raytechniques do provide some measure of success in detecting objectslocated in turbid media, they are not typically well-suited fordetecting very small objects, e.g., tumors less than 1 mm in sizeembedded in tissues, or for detecting objects in thick media. Inaddition, X-ray radiation can present safety hazards to a person exposedthereto. Ultrasound and magnetic resonance imaging (MRI) offeralternatives to the use of X-rays but have their own drawbacks.

Another technique used to detect objects in turbid media, such as tumorsin tissues, is transillumination. In transillumination, visible light isincident on one side of a medium and the light emergent from theopposite side of the medium is used to form an image. Objects embeddedin the medium typically absorb the incident light and appear in theimage as shadows. Unfortunately, the usefulness of transillumination asa detection technique is severely limited in those instances in whichthe medium is thick or the object is very small. This is because lightscattering within the medium contributes to noise and reduces theintensity of the unscattered light used to form the image shadow.

To improve the detectability of small objects located in a turbid mediumusing transillumination, many investigators have attempted toselectively use only certain components of the transilluminating lightsignal. This may be done by exploiting the properties of photonmigration through a scattering medium. Photons migrating through aturbid medium have traditionally been categorized into three majorsignal components: (1) the ballistic (coherent) photons which arrivefirst by traveling over the shortest, most direct path; (2) the snake(quasi-coherent) photons which arrive within the first δt after theballistic photons and which deviate, only to a very slight extent, off astraight-line propagation path; and (3) the diffusive (incoherent)photons which experience comparatively more scattering than do ballisticand snake photons and, therefore, deviate more considerably from thestraight-line propagation path followed by ballistic and snake photons.

Because it has been believed that ballistic and snake photons containthe least distorted image information and that diffusive photons losemost of the image information, efforts to make transillumination workmost effectively with turbid media have focused on techniques whichpermit the selective detection of ballistic and snake photons whilerejecting diffusive photons. This process of selection and rejection hasbeen implemented in various time-gating, space-gating andtime/space-gating techniques. Patents, patent applications andpublications which disclose certain of these techniques include U.S.Pat. No. 5,140,463, inventors Yoo et al., which issued Aug. 18, 1992;U.S. Pat. No. 5,143,372, inventors Alfano et al., which issued Aug. 25,1992; U.S. Pat. No. 5,227,912, inventors Ho et al., which issued Jul.13, 1993; presently-pending and allowed U.S. patent application Ser. No.07/920,193, inventors Alfano et al., filed Jul. 23, 1992; Alfano et al.,"Photons for prompt tumor detection," Physics World, pp. 37-40 (January1992); Wang et al., "Ballistic 2-D Imaging Through Scattering WallsUsing an Ultrafast Optical Kerr Gate," Science, Vol. 253, pp. 769-771(Aug. 16, 1991); Wang et al., "Kerr-Fourier imaging of hidden objects inthick turbid media," Optics Letters, Vol. 18, No. 3, pp. 241-243 (Feb.1, 1993); Yoo et al., "Time-resolved coherent and incoherent componentsof forward light scattering in random media," Optics Letters, Vol. 15,No. 6, pp. 320-322 (Mar. 15, 1990); Chen et al., "Two-dimensionalimaging through diffusing media using 150-fs gated electronic holographytechniques," Optics Letters, Vol. 16, No. 7, pp. 487-489 (Apr. 1, 1991);Duncan et al., "Time-gated imaging through scattering media usingstimulated Raman amplification," Optics Letters, Vol. 16, No. 23, pp.1868-1870 (Dec. 1, 1991), all of which are incorporated herein byreference.

Of the above-listed art, Wang et al., "Kerr-Fourier imaging of hiddenobjects in thick turbid media," Optics Letters, Vol. 18, No. 3, pp.241-243 (Feb. 1, 1993) is illustrative. In this article, there isdisclosed a time/space-gating system for use in imaging opaque test barshidden inside a 5.5 cm-thick 2.5% Intralipid solution. The disclosedsystem includes three main parts: a laser source, an optical Kerr gateand a detector. The laser source is a picosecond mode-locked lasersystem, which emits a 1054 nm, 8 ps laser pulse train as theillumination source. The second harmonic of the pulse train, which isgenerated by transmission through a potassium dihydrate phosphate (KDP)crystal, is used as the gating source. The illumination source is sentthrough a variable time-delay and is then used to transilluminate, fromone side, the turbid medium containing the opaque object. The signalfrom the turbid medium located at the front focal plane of a lens iscollected and transformed to a Kerr cell located at its back focal plane(i.e., the Fourier-transform spectral plane of a 4F system). Thatportion of the Kerr cell located at the focal point of the 4F system isgated at the appropriate time using the gating source to preferentiallypass the ballistic and snake components. The spatial-filtered andtemporal-segmented signal is then imaged by a second lens onto a CCDcamera.

Although time- and/or space-gating techniques of the type describedabove have provided a modicum of success in improving transilluminatedimages, there still remains considerable room for improvement.

It has long been known that the accumulation of ice and/or snow on anylifting or control surface of an aircraft, such as an airplane wing oron a helicopter rotor, can lead to disastrous results. Accordingly,considerable effort has been expended in the past to devise techniquesthat enable the detection of ice and/or snow on airplane wings andsimilar surfaces. At present, a variety of ice detection techniquesexist which have had varying degrees of success. Some such techniquesrely on the thermal detection of ice, others on the electrical orultrasonic detection of ice. Still other techniques, such as thosedisclosed in U.S. Pat. No. 5,500,530, inventor Gregoris, which issuedMar. 19, 1996, U.S. Pat. No. 5,484,121, inventors Padawer et al., whichissued Jan. 16, 1996, U.S. Pat. No. 5,400,144, inventor Gagnon, whichissued Mar. 21, 1995, U.S. Pat. No. 5,296,853, inventors Federow et al.,which issued Mar. 22, 1994, and U.S. Pat. No. 5,180,122, inventorsChristian et al., which issued Jan. 19, 1993, all of which areincorporated herein by reference, rely on optical or electro-opticaldetection techinques.

In U.S. Pat. No. 5,475,370, inventor Stern, which issued Dec. 12, 1995,and which is herein incorporated by reference, there is disclosed asystem for detecting the presence of an energy polarization alteringdielectric material, such as ice or snow, on a surface, such as part ofan aircraft, which normally specularly reflects incident energy, such aslight, when there is no such dielectric present. The energy is conveyedfrom a transmitter along a path to the surface and the incident energyis reflected from the surface along a path to a receiver with adielectric on the surface destroying any polarization, such as circular,of the energy and that reflected from a specular portion maintaining thepolarization. An optical system in one or both of the paths operates inan isolator state to produce an image of the dielectric portion having afirst intensity level and that of the specular portion passing throughthe optical system having a different intensity level. When the opticalsystem is operated alternately in isolator and non-isolator states itproduces an image of the dielectric portion having a relatively steadyintensity level and that of the specular portion alternating betweenfirst and second different intensity levels corresponding to theisolator and non-isolator states of the optical system.

One problem noted by the present inventors with the technique of theabove-identified Stern patent is that, because the Stern technique isbased on the depolarization of specularly reflected light (with icebeing treated as a dielectric material that destroys any polarizationwhile metal maintains polarization), appropriate alignment of the planeof reflection of the object must be maintained with respect to theposition of the illuminating source and the detector so that thespecularly reflected light from the object is directed to the detector.

SUMMARY OF THE INVENTION

The present invention is based, in part, on the discovery that, when apulse of initially-polarized light is used to illuminate a turbidmedium, such as a human tissue, ice or snow, the ballistic andsnake-like components of the light, which are either backscatteredrelatively directly from the surface of the turbid media or take directpaths through the turbid media, substantially maintain the polarizationof the initially polarized light while the diffuse component of thelight, which tends to travel longer, less direct paths through theturbid media before either being backscattered from the turbid media oremerging from the opposite end of the turbid medium, becomescompartively more depolarized than do the ballistic and snake-ikecomponents. Moreover, even where the pulse of light is initiallyunpolarized, the above discovery can be made use of since, when a pulseof initially unpolarized light is used to illuminate a turbid medium,such as a human tissue, the initially unpolarized light becomespartially polarized.

Therefore, according to one aspect, the present invention relates to amethod for imaging the surface of a turbid medium, said methodcomprising the steps of: (a) illuminating the surface of the turbidmedium with light, whereby light is backscattered from the illuminatedsurface of the turbid medium; (b) detecting a pair of complementarypolarization components of the backscattered light; and (c) forming animage of the illuminated surface using the pair of complementarypolarization components.

According to another aspect, the present invention relates to a methodfor imaging an object located in or behind a turbid medium, said methodcomprising the steps of: (a) illuminating an object in or behind aturbid medium with light, whereby light is backscattered from the objectin or behind the turbid medium; (b) detecting a pair of complementarypolarization components of the backscattered light; and (c) forming animage of the object using the pair of complementary polarizationcomponents.

Preferably, the illuminating light is polarized (e.g., linearlypolarized, circularly polarized, elliptically polarized) and may becontinuous wave or pulsed. Where, for example, the illuminating light islinearly polarized, the pair of complementary polarization componentsare preferably the parallel and perpendicular components to thepolarized illuminating light, and the image may be formed by subtractingthe perpendicular component from the parallel component, by taking aratio of the parallel and perpendicular components or by using somecombination of a ratio and difference of the parallel and perpendicularcomponents.

One possible application of the present invention is in "opticalfingerprinting." An image of the fingerprint pattern of a person'sfinger or the like may be obtained, for example, by illuminating anappropriate region of a finger with a pulse of linearly polarized light,detecting the parallel and perpendicular components of the backscatteredlight, subtracting the perpendicular component from the parallelcomponent to form a difference and forming an image of the illuminatedregion of the finger using said difference. This image may then bedigitally recorded using a video or CCD camera and stored in a databaseto permit search and/or comparison of unidentified images withidentified images in the database.

The present invention is also based, in part, on the discovery that onecan image a turbid medium at various depths thereof by illuminating theturbid medium with light pulses of different wavelengths and using adifference, ratio or some combination thereof of the respectiveperpendicular components to form an image.

Consequently, according to yet another aspect, the present inventionrelates to a method for imaging a turbid medium at a depth below thesurface thereof, said method comprising the steps of: (a) illuminatingthe turbid medium with a first light of a first wavelength, the firstlight being polarized and having a first state of polarization, wherebysaid first light, after entering the turbid medium, emerges therefrompartially depolarized; (b) detecting a component of the partiallydepolarized first light that is normal to said first state ofpolarization; (c) illuminating the turbid medium with a second light ofa second wavelength, said second wavelength being different from saidfirst wavelength, the second light being polarized and having a secondstate of polarization, whereby said second light, after entering theturbid medium, emerges therefrom partially depolarized; (d) detecting acomponent of the partially depolarized second light that is normal tosaid second state of polarization; and (e) forming an image of theturbid medium using the normal components of the partially depolarizedfirst and second light.

The first and second polarized illuminating light may be, for example,pulses of linearly polarized light, and the image may be formed bysubtracting the perpendicular component obtained from one pulse from theperpendicular component obtained from the other pulse, by taking a ratioof the respective perpendicular components or by using some combinationof a ratio and difference of the respective perpendicular components.

The present invention is also based, in part, on the discovery thatdifferent types of materials, such as diffusively reflective metals onone hand and ice and/or snow on the other hand, depolarize polarizedlight to different extents and that, therefore, the presence of iceand/or snow on airplane wing or a similar structure can be detected byobserving the extent to which polarized light used to illuminate anairplane wing or the like becomes depolarized. Moreover, even where thelight is initially unpolarized, the above discovery can be made use ofsince, when initially unpolarized light is backscattered off the surfaceof a metal or dielectric, the two polarization components of thebackscattered light differ in intensity.

Consequently, according to yet another aspect, the present inventionrelates to a method for detecting snow or ice on an airplane wing or thelike, said method comprising the steps of: (a) illuminating an airplanewing with light, whereby light is diffusively backscattered from theilluminated airplane wing; (b) detecting a pair of complementarypolarization components of the diffusively backscattered light; and (c)using the pair of complementary polarization components to determinewhether snow or ice is present on the illuminated airplane wing.

The technique of the present invention is, therefore, to be contrastedwith the above-discussed Stern technique in that, in the presenttechnique, ice and metal are treated as light scattering objects anddiffusively reflected light, as opposed to specularly reflected light,is used. In fact, the present inventors have found the use of specularlyreflected light in the Stern technique to be troublesome in that itoften creates "hot areas" in images.

The present invention is also directed to apparatuses for performing theabove-described methods.

Additional objects, features, aspects and advantages of the presentinvention will be set forth in part in the description which follows,and in part will be obvious from the description or may be learned bypractice of the invention. Various embodiments of the invention will bedescribed in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that changes may be made withoutdeparting from the scope of the invention. The following detaileddescription is, therefore, not to be taken in a limiting sense, and thescope of the present invention is best defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are hereby incorporated into andconstitute a part of this specification, illustrate various embodimentsof the invention and, together with the description, serve to explainthe principles of the invention. In the drawings wherein like referencenumerals represent like parts:

FIG. 1 is a graphic representation of temporal profiles of the parallelpolarization component, perpendicular polarization component andnormalized perpendicular polarization component of backscattered lightobtained from bovine gray matter brain tissue illuminated with 1064 nm,6.5 ps laser pulses;

FIGS. 2(a) through 2(c) are images of a human palm illuminated with 580nm polarized laser pulses and obtained using (a) the parallelpolarization image component; (b) the perpendicular polarization imagecomponent; and (c) the parallel polarization image component minus theperpendicular polarization image component, respectively, of thebackscattered light;

FIGS. 3(a) and 3(b) are images of the "fingerprint pattern" of the skinof a human palm obtained using 630 nm illumination and (a) conventionalimaging (no polarizers involved); and (b) polarization differenceimaging (I_(parallel) -I_(perpendicular));

FIGS. 4(a) through 4(c) are images of the back of a human hand formedusing (a) the perpendicular polarization component backscattered fromthe hand following 570 nm illumination; (b) the perpendicularpolarization component backscattered from the hand following 600 nmillumination; and (c) the difference obtained by subtracting theperpendicular polarization component of (a) from (b);

FIG. 5 is a graphic representation of the respective perpendicularpolarization components backscattered from bovine gray and white matterfollowing 1064 nm, 6.5 ps illumination;

FIGS. 6(a) and 6(b) are graphic representations of the parallel andperpendicular polarization components detected from backscattered lightfollowing 532 nm, 4 ps laser pulse illumination of (a) an aluminum plateand (b) 1 mm ice deposited on the aluminum plate, respectively;

FIGS. 7(a) through 7(e) are images of a square copper plate where four1.25 cm diameter holes of different depths (0.25 mm, 0.5 mm, 1 mm and 2mm) are filled with ice, the plate being illuminated with linearlypolarized laser light at 632.8 nm, a cooled CCD camera being used torecord the image, and (a) no polarizer being positioned in front of theCCD camera, (b) a polarizer being positioned in front of the CCD, thepolarizer being oriented parallel to the polarization of theilluminating light, (c) a polarizer being positioned in front of theCCD, the polarizer being oriented perpendicular to the polarization ofthe illuminating light, (d) the image being obtained from the differencebetween the parallel and perpendicular components (I.sub.∥ -I.sub.⊥),and (e) the image being obtained from the difference between theparallel and perpendicular components divided by the perpendicular image(I.sub.∥ -I.sub.⊥)/I.sub.⊥ !;

FIG. 8 is a schematic view of a first embodiment of an imaging systemconstructed according to the teachings of the present invention, theimaging system being shown used in a medical application;

FIG. 9 is a schematic view of the imaging system of FIG. 8 being used ina non-medical application;

FIG. 10 is a schematic view of a second embodiment of an imaging systemconstructed according to the teachings of the present invention, theimaging system being adapted for either medical or non-medicalapplications;

FIGS. 11 (a) and 11(b) are schematic diagrams illustrating theimprovement in observation depth in turbid media obtainable using thetechnique of the present invention;

FIG. 12 is a schematic view of a third embodiment of an imaging systemconstructed according to the teachings of the present invention, theimaging system being particularly well-adapted for detecting ice onairplanes; and

FIG. 13 is a schematic view of a fourth embodiment of an imaging systemconstructed according to the teachings of the present invention, theimaging system being particularly well-adapted for detecting ice onairplanes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to a method that enables high resolutionand high-speed optical imaging of objects located in, at the surface of,or behind highly scattering media using polarized illuminating light andpolarization-difference imaging. Using the present invention, surface,as well as beneath-the-surface, imaging of tissues and other turbidmedia can be achieved in a backscattered geometry. The surface imageinformation is predominately carried by the parallel polarization imagecomponent while the perpendicular image component containsbeneath-the-surface image information. Images of structures at differentdepths within a turbid medium can be obtained using the perpendicularpolarization component resulting from the use of different illuminatingwavelengths.

The present invention also relates to a method that enables improvedoptical image quality and visibility of object features based on theprinciple that, when an object is illuminated with polarized light, theparallel component of light scattered by the object is more intense thanthe perpendicular component thereof. Using this principle, ice on thewings of a plane or mines in the sea can be detected by illuminatingwith polarized light, detecting the resultant parallel and perpendicularpolarization components, and subtracting the perpendicular polarizationcomponent from the parallel polarization component, any changes indepolarization likely being caused by the presence of ice on the wing ora mine in water. In addition, the images of objects in smoke, fog orsmog can be made more visible by obtaining the parallel andperpendicular components in the manner discussed above and subtractingthe perpendicular component from the parallel component so as tocancel-out the effects of the diffusive photons responsible forobscuring the image.

Referring now to FIG. 1, there is shown a graphic representation of theparallel (I.sub.(t)∥), perpendicular (I.sub.(t)⊥), and normalizedperpendicular (I.sub.(t)⊥) components of backscattered light detectedfrom a sample of bovine gray matter brain tissue using the followingexperimental setup: The laser system used was a synchronously-pumpedtunable R-6G dye laser pumped by a mode-locked, pulse-compressed Nd:YAGlaser. The 6.5 ps, 1064 nm laser pulses and a streak camera with a fiberprobe were used to record the temporal profiles of the backscatteredlight. For the imaging measurements, the dye laser beam tunable in the570-635 nm spectral region was used as the illuminating source withaverage power of about 5 mW. A cooled CCD camera and imaging optics wereused to record the parallel and perpendicular image components withexposure time of 200 msec.

FIG. 1 shows the polarization components of the backscattered light whenlinearly polarized 1064 nm pulses were used. The diameter of theilluminating beam was 0.5 mm. As can be seen in FIG. 1, the parallelpolarization component (I.sub.(t)∥) is more intense than theperpendicular polarization component (I.sub.(t)⊥). In addition, when thetemporal profile of the perpendicular component was normalized to thepeak intensity of the parallel component (I_(norm)(t)⊥), the temporalprofiles were different, having different peak time positions and fourwave harmonic mixing (FWHM). The scattering parameters of gray matterallow for clear observation of the above mentioned differences in thetwo backscattering polarization components within the temporalresolution of our system (about 15 ps). Similar differences are presentin different types of tissue but with different time profiles and peakintensities.

Without wishing to be limited to any theory, it is believed that thereason for the differences in the two polarization components arisesfrom the fact that the backscattered photons that are perpendicularlypolarized must first undergo sufficient scattering events to lose theirpolarization information. By contrast, the photons that are directlybackscattered from the surface of the tissue have less of an opportunityto depolarize; therefore, the backscattering photons from the surface ofthe tissue and initial layers beneath the surface belong mostly to theparallel polarization image component. However, the perpendicularpolarization component contains predominately photons that penetratedthe tissue to certain depths before they emerged in the backscatteringdirection. This salient concept explains the differences in the temporalprofiles of the two polarization components shown in FIG. 1. Theparallel component contains all photons backscattered from the surfaceof the tissue before they depolarize. As a result, the parallelcomponent is more intense and the time of peak intensity is when thebackscattered light from the surface arrives. The perpendicularcomponent time of peak intensity arises from the depolarized photonsthat propagated the tissue to a certain depth and then backscattered.

The above results suggest that image information of the surface andnear-surface of tissues is contained predominately in the parallelpolarization image component while the perpendicular polarizationcomponent contains information of that portion of the tissue wellbeneath the surface. This concept is demonstrated in FIGS. 2(a) through2(c) where a 580 nm linearly polarized laser beam was used to illuminatethe palm of a human hand and a CCD detector was used to record the twopolarization image components (i.e., parallel and perpendicular). FIG.2(a), which is the parallel polarization image, shows detailedstructures of the skin. FIG. 2(b), which is the perpendicularpolarization image, shows no skin structures. The gray shades in FIG.2(b) are due to the differing blood concentrations under the skin. Theparallel image component contains photons backscattered from the skinand after penetrated underneath the skin while the perpendicular imagepredominately contains photons that have penetrated underneath the skin.Consequently, subtracting the two image polarization components, aclearer image of the skin can be obtained. This is demonstrated in FIG.2(c), where the image is obtained by direct subtraction of the twopolarization components (FIGS. 2(a) and 2(b)). The image componentarising from photons that propagated deep enough into the tissue andlost their polarization information is equally split between the twoimage polarization components. By subtracting the two image polarizationcomponents, the depolarized image component arising from the photonsthat propagated through the skin before retroreflected should cancelout. The image obtained is formed by photons that are still polarizedbecause they have undergone less scattering; consequently, theypredominately belong to the photons backreflected from the surface orslightly beneath the surface. This process leads to an image whichcontains detailed information about the surface of the skin structures,as demonstrated in FIG. 2(c).

Referring now to FIGS. 3(a) and 3(b), it can be seen how the techniqueof the present invention can be used to obtain images of the"fingerprint pattern" of fingers, palms and the like that are superiorto those that would be obtained using conventional imaging techniques.FIG. 3(a) is an image of a portion of a human palm obtained usingpolarized 630 nm illumination and conventional imaging of backscatteredlight (no polarizers involved). FIG. 3(b) is an image of the same objectusing polarized 630 nm illumination wherein the parallel andperpendicular components of the backscattered light are detected (usingpolarizers and a video system, CCD camera or the like), theperpendicular component is subtracted from the parallel component toyield a polarization difference and the polarization difference image isshown. As can be seen, FIG. 3(b) is much improved over FIG. 3(a) interms of image quality and contrast. The image of FIG. 3(b) could beused for "optical fingerprinting" in which the image is digitallyrecorded using a video or CCD camera and stored in a database to permitsearch and/or comparison of unidentified images with identified imagesin the database. Alternatively, as will be better understood in view ofthe discussion below, one could illuminate a sample with polarized lightand use the perpendicular image component only to obtain images freefrom surface image information or to obtain images free from specularreflection.

The time profile of the backscattered pulse is given by the followingrelation:

    J(μ'.sub.s,μ.sub.a,t)=I.sub.0 S(μ'.sub.s,t)exp(-μ.sub.a ct)

where μ'_(s) is the transport scattering coefficient, μ_(a) is theabsorption coefficient, I₀ is the intensity of the injected pulse whileS(μ'_(s),t) and exp(-μ_(a) ct) represent scattering and absorption,respectively. The temporal profile of the retroreflected pulse stronglydepends on μ_(a). For larger μ_(a), the FWHM of the backscattered pulseis smaller, indicating a shorter average distance of flight <L> of therecorded photons. The mean photon-visit depth z of the backscatteredphotons depends on μ_(a), as well as μ'_(s), while in a time resolvedmeasurement, the dependence of z(t) is nearly linear to the square rootof the detection time. This means that using different illuminatingwavelengths, the decay time of the temporal profile of the backscatteredlight pulse and, consequently, z can be appropriately adjusted bychanging μ'_(s) and μ_(a). In addition, FIGS. 1 and 2(a) through 2(c)demonstrate that the perpendicular polarization component containsphotons that propagated inside the tissue before being backscattered andare almost free from scattering from the surface.

Based on the above concepts, it can be appreciated that structureslocated at different depths inside a tissue can be imaged using thefollowing principles: (a) different illuminating wavelengths can be usedto reach different mean photon-visit depths z(t); (b) the perpendicularpolarization image component can be used to avoid the image informationfrom the surface of the tissue; (c) the intensity and time of exposurecan be adjusted so that using the same parameters in pulsed illuminationone can have equal peak intensities of the temporal profiles of thebackscattered pulses for the different wavelengths; and (d) imagesobtained under different illuminating wavelengths can be subtracted toobtain a new image of structures underneath the surface. The imagesobtained in this way contain information from different depths z but,due to normalization, images from the front part of the tissue cancelout, and the remaining image arises from the photons that propagateddeeper into the tissue in the longer wavelength image. Consequently, byselecting appropriate illuminating wavelengths, one can effectivelyreach different "depth zones" inside the tissue.

Referring now to FIGS. 4(a) through 4(c), the results of theaforementioned technique can be seen. The image in FIG. 4(a) shows theperpendicular image component under 570 mn illumination of the back of ahand. Due to high absorption by the hand tissue at this wavelength, thephotons backscattered do not penetrate the tissue much and, as a result,only a scratch on the skin located in the lower right corner of FIG.4(a) can be seen as a dark line. FIG. 4(b) shows the perpendicularpolarization image under 600 nm illumination at the same position. Theabsorption of blood at 600 nm is about 10 less than at 570 nm, allowingfor a deeper propagation of the 600 nm photons before beingbackscattered. The scratch at the lower right side cannot be seen due tothe fact that it is superficial and its presence is masked by photons at600 nm that have propagated deeper into the tissue. However, one canbarely see in FIG. 4(b) some veins located underneath the skin.Subtraction of the 570 nm image from the 600 nm image leads to an imagearising from photons that propagate to deeper layers that were reachedby the 600 photons but not by the 570 nm photons. This principle isdemonstrated in FIG. 4(c), where the veins under the skin are clearlyobservable as darker structures with actual size about 1 mm while thescratch is shown in the lower right corner as a structure of brighterintensity. The exposure time/illuminating intensity for the two imageswas appropriately adjusted so that the image components from the outerpart of the tissue were cancelled-out during image subtraction. Theexposure parameters were obtained using the method described above sothat the peak intensities of the time profiles under 570 and 600 nmpulsed illumination were equal.

The aforementioned imaging technique of image polarization subtractionat different wavelengths can be used to highlight differences inabsorption by blood or scattering due to the presence of different typesof tissues at different depths using differences in μ_(a) and μ'_(s).Referring now to FIG. 5, there is shown the backscattered perpendicularpolarization components of bovine gray and white matter, respectively,using 1064 nm, 6.5 ps pulsed illumination. The perpendicularpolarization component of white matter is more intense than that of thegray matter because of higher scattering and depolarization. Thedifference in scattering and depolarization from different types oftissues enables different structures inside the tissue to be observedusing the polarization difference technique of the present invention.With gradual changes of the illuminating wavelengths (and adoptingappropriate optical techniques, such as confocal microscopy), one may beable to display histological structures at different depths. The maximumdepth at which imaging may be achieved will depend on the scattering andabsorption characteristics of the particular tissues involved.

Therefore, it can be appreciated that optical polarization imaging ofthe type herein described can be used to obtain images at surfaces andat different depths. The surface of a tissue can be highlighted andimaged better by subtraction of the two image polarization components atthe same illuminating wavelength. Images underneath the skin can beobtained by subtraction of the perpendicular polarization components atdifferent illuminating wavelengths. This technique can be used for invivo imaging of the skin, mucosa, vascular and arterial systems, GYN andgastrointestinal track, with relatively simple equipment and high speedof image formation that will allow imaging in real time.

When polarized light is scattered by an object (i.e., diffusereflection), the two polarization components (parallel and perpendicularto the initial polarization state) of the scattered light are not equalin intensity due to differences in scattering and reflection by theobject. The backscattered light can be separated into two segments, thestill polarized segment and the depolarized segment. The still polarizedsegment belongs to the parallel polarization component of thebackscattered light while the depolarized segment is evenly distributedbetween the two polarization components. As a result, the parallelpolarization component of the backscattered light is more intense thanthe perpendicular. The degree that light depolarizes when scattered byan object depends on the optical properties of the object. For example,scattering by a metallic object depolarizes light very little whereasscattering by an object which light is capable of penetrating and beingmultiply scattered internally before exiting the medium (e.g.,biological systems, ice, snow, etc.) depolarizes light much more.

A pulse-compressed Nd:YAG laser emitting 532 nm, 4 ps pulses was used toilluminate an aluminum plate and ice deposited on the same aluminumplate. Backscattered light was collected using a fiber probe, and thetemporal profile of the parallel and perpendicular polarizationcomponents were recorded by a streak camera with 10 ps time-resolution.The recorded time profiles of the two polarization components of thebackscattered pulse for aluminum and ice are shown in FIGS. 6(a) and6(b), respectively. As can be seen in FIG. 6(a), the backscatteredparallel polarization component for aluminum (airplane wing) is muchmore intense than the perpendicular polarization component. However, ascan be seen in FIG. 6(b), when ice 1 mm thick on the aluminum plate (tosimulate ice on the wing of an airplane) was illuminated with polarizedlight, the difference in intensity between the two polarizationcomponents was significantly smaller and the relative intensity of thetwo backscattered polarization components was very different than forthe bare aluminum plate.

Additional imaging and detection of ice on a metal surface is shown inFIGS. 7(a) through 7(e). In a square copper plate there are four 1.25 cmdiameter holes of different depth (0.25, 0.5, 1 and 2 mm) which arefilled with ice. The target is illuminated with linearly polarized laserlight at 632.8 nm and a cooled CCD camera is used to record the imageformed by the photons scattered off the ice or metal surface. The metalsurface was slightly rotated so that no specularly reflected lightreaches the CCD camera. Specularly reflected light is causing theformation of "hot spots" in the image which may inhibit the detection ofice or make it more difficult to detect. FIG. 7(a) shows the image ofthe target with no polarizer in front of the CCD. The four circularholes filled with ice can be seen aligned at the corners of an innersquare. FIG. 7(b) shows the parallel polarization image of the target(polarizer in front of the CCD is parallel with the polarization of theilluminating light) while FIG. 7(c) shows the perpendicular imagecomponent. As discussed in the previous paragraph, the ice in theparallel image appears darker than the metal while in the perpendicularimage, the ice appears brighter than the metal. FIG. 7(d) shows theimage obtained after subtraction of the two image components while FIG.7(e) represents the (I_(parallel) -I_(perpendicular))/I_(perpendicular)image. In the resulting images shown in FIGS. 7(d) and 7(e), the ice isrecorded with much less intensity than the metal (FIG. 7(e) provides thehighest contrast) demonstrating the usefulness of this technique for icedetection and imaging.

Based on the above, a polarization-difference under polarizedillumination imaging system is herein described that enables enhancedvisibility of target-object features in a light-scattering environmentor in a light-transparent environment. The designing principles of thissystem are as follows: (1) polarized light is used for the illuminationof the target; (2) the two image polarization components are recorded;and (3) the perpendicular image component is subtracted from theparallel image component to obtain the final image of the object.

The first of the above requirements can be satisfied in some cases evenif the primary light source is unpolarized. It is known that whenunpolarized light is reflected off the surface of a metal or adielectric, the two polarization components of the reflected light aredifferent in intensity depending upon the wavelength and the angle ofincidence of the illuminating light (Fresnel's laws of reflection). As aresult, the light reflected off the metal or dielectric object becomespartially polarized. In other instances, optical anistropy of an objectcauses an initially unpolarized light illuminating the object to becomepartially polarized when reflected off the object. In still otherinstances, when unpolarized light propagates through the interface oftwo dielectrics, it becomes partially polarized and, therefore, anobject inside the second dielectric is illuminated with partiallypolarized light. The optical polarization difference method of thepresent invention can, therefore, be used in all of the above instancesby recording the two polarization image components and calculating thedifference between the two components (or performing some otherinterimage operation on the two components) to obtain the opticalpolarization difference (OPDI) image of the object even though theprimary illuminating source was not polarized.

The optical polarization difference technique of the present inventioncan be used in a variety of applications where the scattering of lightby the environment inhibits clear observation of the target-object orwhen target features need to be revealed. The present technique ishereinafter described in two contexts: (a) imaging when an object issurrounded by an intense scattering medium; and (b) detection of objectfeatures.

When polarized light travels through a scattering medium (such as fog,smog, smoke, etc.), a gradual process of depolarization takes place. Thelight completely loses its polarization only after traveling a longdistance into the smoke or fog or smog. When partially polarized lightilluminates the object, the two polarization components of the reflectedlight are different in intensity with the parallel component moreintense than the perpendicular. Similarly, unpolarized light reflectedoff an object can give rise to different intensities for the twopolarization components (defined by the plane of incidence of the lightor the optical anisotropy axis of the material) as discussed above. Asthe partially polarized reflected light from the object travels throughsmoke or the like towards a detection system it continues to bedepolarized. The light that reaches the detection system, therefore, hasbeen depolarized greatly due to scattering in the smoke and due toscattering in the object. However, the still polarized photons arecarrying the image information even though their relative intensity withrespect to the diffusive-unpolarized photons decreases. As a result, theimage quality decays and the image is lost in the "white" backgroundmade up of the unpolarized diffusive light. Subtraction of the twopolarization image components cancels-out the depolarized component aswell as the background light (which is unpolarized), and the generatedpolarization-difference image is made up of the still polarized photonsthat have undergone less scattering in the smoke and contain the imageinformation. In this way, the "white" background from the diffusivephotons is subtracted and the image is revealed again. The visibilityinside the fog or smog or smoke is improved while the range at which theimage is observable depends on the density of the smoke or fog or smogas well as on the detection system. In general, as long as there arepolarized photons reaching the detector, an image of the object can bereconstructed using polarization difference imaging.

The technique of the present invention can be used to increase theobservation depth of a fireman entering a building or space filled withsmoke. Similarly, the present technique can be used to help a driver ofa car or train or the pilot of a plane to see better and in greaterdepth in fog.

The present invention can also be used to detect ice on the wings of aplane in the following manner: Polarized light is used to illuminate awing of a plane. The two polarization image components scattered by thewing are detected and the difference between them is calculated toobtain a polarization-difference image. The background light componentcancels-out because it is evenly distributed in both polarization imagecomponents. The polarized light scattered by the ice-free portions ofthe aluminum wing remains almost completely polarized, as in FIG. 6(a).However, the light scattered by the ice-covered part of the wing isstrongly depolarized, as in FIG. 6(b). As a result, the ice-covered partof the plane in the perpendicular image component is shown having ahigher intensity than that for the ice-free part of the plane whereasthe parallel image component is shown having a lower intensity than thatfor the ice-free part of the plane. Consequently, thepolarization-difference image shows the ice-free part of the wing withhigh intensity because of the big difference in intensity between theparallel and perpendicular components scattered by the ice-free part ofthe wing, and the ice-covered part of the wing with low intensitybecause of the small difference in intensity between the parallel andperpendicular components scattered by the ice-covered part of the wing.

Based on the differences in depolarization that take place whenpolarized light is scattered by the wing of a plane when ice is or isnot present thereon, a number of different techniques can be used todetect ice on a wing. For example, the detection of ice can be performedusing a polarization-difference image of the type described above.Alternatively, the difference of the two polarization image components(I.sub.∥ -I.sub.⊥) can be replaced by any image operation containingI.sub.∥ and I.sub.∥, such as I.sub.∥ /I.sub.⊥, I.sub.∥ -I.sub.⊥ !/I.sub.∥ +I.sub.⊥ !, I.sub.⊥ !/ I.sub.∥ -I.sub.⊥ !, I.sub.∥ -I.sub.⊥ !/I.sub.⊥ ! and I.sub.∥ !/ I.sub.∥ -I.sub.⊥ !.

Detection of ice can also be performed by scanning the wing of the planewith a polarized light beam and detecting point-by-point thedepolarization of the scattered light. The intensities of the twopolarization components of the initially polarized beam aresimultaneously measured and the ratio of the parallel over theperpendicular intensities from each point on the wing (I.sub.∥ /I.sub.⊥)is calculated. When the ratio is high (I.sub.∥ /I.sub.⊥)>>1!, the wingis free of ice. When the ratio approaches 1 1<(I.sub.∥ /I.sub.⊥)<2!,there is ice on the wing. In the point-by-point scanning technique,instead of using the ratio (I.sub.∥ /I.sub.⊥), one may choose to useother finctions, such as (I.sub.∥ -I₁₉₅ ), I.sub.∥ -I.sub.⊥ !/ I.sub.∥+I.sub.⊥ !, I.sub.⊥ !/ I.sub.∥ -I.sub.⊥ !, I.sub.∥ -I.sub.⊥ !/ I.sub.⊥ !and I.sub.∥ !/ I.sub.∥ -I.sub.⊥ !.

The foregoing technique can be used in a variety of applications. Forexample, the detection of mines in the sea can be achieved using theabove method. The scattering and depolarization of polarized light by amine will provide the basis for detection of the mine. Sunlight may beused as the illuminating source, which will be partially polarized underthe water depending upon its angle of incidence with respect to the sea.Subtraction of the two polarization image components will reveal thechange in depolarization in the field of observation introduced by themine and will pinpoint the location of the mine. The present techniquecan also be used to detect target features when the target exhibitsoptical anisotropy.

Referring now to FIG. 8, there is shown a schematic view of a firstembodiment of an imaging system constructed according to the teachingsof the present invention, the imaging system being represented generallyby reference numeral 11.

System 11, which may be used, as in FIG. 8, to image tissue samples T,comprises an illuminating light source 13. Light source 13 may be alaser, a lamp or the like. System 11 also includes a rotatably-mountedpolarizer 15, which is used to ensure that the light from source 13 ispolarized. System 11 further includes imaging optics 17 for imagingbackscattered light from tissue sample T onto a 2-dimensional cooled CCDcamera 19. A rotatably-mounted analyzer 21, which is used to select theparallel and perpendicular components of the backscattered light, ispositioned between optics 17 and camera 19. As can readily beappreciated, to enable the detection of both the parallel andperpendicular components of the backscattered light, one may either keeppolarizer 13 in the parallel position while sequentially placinganalyzer 21 in the parallel and perpendicular positions or vice versa.

System 11 also includes 23 a computer for analyzing the informationdetected by detector 19 regarding the parallel and perpendicularcomponents of the backscattered light in the manner discussed above anda monitor 25 for displaying an image of the illuminated tissue sample inaccordance with the data outputted by computer 23.

Referring to FIG. 9, system 11 can be seen being applied to non-medicalimaging.

Referring now to FIG. 10, there is shown a second embodiment of animaging system constructed according to the teachings of the presentinvention, the imaging system being represented generally by referencenumeral 51.

System 51 includes an illuminating source 53, which may be the same asilluminating source 13 of system 11. System 51 also includes a rotatablymounted polarizer 54, which is used to ensure that polarized light isinputted into a polarization preserving fiber 55. Fiber 55, in turn, isdisposed within a working channel 57 of an endoscope 59 and may be usedto illuminate a tissue sample T. The backscattered light from tissuesample T is collected by an image-collection fiber bundle 61 disposedwithin endoscope 59. A rotatably-mounted analyzer 63 is located at thedistal end of bundle 61 and is used to select the parallel andperpendicular components of the backscattered light. (As can readily beappreciated, either polarizer 54 can be placed in the parallel positionwhile analyzer 63 is placed in the parallel and perpendicular positionsor vice versa.) The light passed through analyzer 63 is then detected bya detector 65, the output of which is then transmitted to a computer 67.A monitor 69 is coupled to computer 67 for displaying the image.

Referring now to FIGS. 11(a) and 11(b), there is schematically shown theimprovement in observation depth made possible using the technique ofthe present invention. As seen in FIG. 11(a) (where the technique of thepresent invention is not employed), the test mark (A) can barely be seenby the naked eye of an observer whereas, as seen in FIG. 11(b) (wherethe technique of the present invention is employed), the test mark (A)can be seen clearly, and observation depth increases to reach test mark(B).

Referring now to FIGS. 12 and 13, there are shown schematic views of athird and a fourth embodiment of an imaging system constructed accordingto the teachings of the present invention, the imaging systems of FIGS.12 and 13 being particularly well-adapted for detecting ice onairplanes. In the system of FIG. 12, large sections of the airplane arescanned at one time whereas, in the system of FIG. 13, a polarized lightbeam illuminates locally the wing of the plane while a scanner is usedto cover the desired area of the plane point-by-point. The scatteredlight collected passes through a polarizing beam splitter (or otherpolarization selection apparatus) to select the two polarizationcomponents and record their intensity. The polarized light coming fromthe free-of-ice aluminum wing will remain ahnost completely polarizedwhereas the ice-covered part of the wing will strongly depolarize thescattered light. The ratio of the parallel over perpendicularintensities for each point on the wing (I.sub.∥ /I.sub.⊥) is calculated.When the ratio is high, i.e. much greater than 1, the wing is free ofice. When the ratio is nearly 1, i.e., between 1 and 2, then there isice on this part of the wing. Instead of using the ratio (I.sub.∥/I.sub.⊥), one may choose to use other functions such as I.sub.∥-I.sub.⊥ !, I.sub.∥ -I.sub.⊥ !/ I.sub.∥ +I.sub.⊥ !, I.sub.⊥ !/ I.sub.∥-I.sub.⊥ !, I.sub.∥ -I.sub.⊥ !/ I.sub.⊥ ! and I.sub.∥ !/ I.sub.∥-I.sub.⊥ !.

The embodiments of the present invention recited herein are intended tobe merely exemplary and those skilled in the art will be able to makenumerous variations and modifications to it without departing from thespirit of the present invention. All such variations and modificationsare intended to be within the scope of the present invention as deemedby the claims appended hereto.

What is claimed is:
 1. A method for imaging the surface of a turbidmedium, said method comprising the steps of:(a) illuminating the surfaceof the turbid medium with light, whereby light is backscattered from theilluminated surface of the turbid medium; (b) detecting a pair ofcomplementary polarization components of the backscattered light; and(c) forming an image of the illuminated surface using the pair ofcomplementary polarization components.
 2. The method as claimed in claim1 wherein the illuminating light comes from a pulsed or continuous lightsource (lamp, laser).
 3. The method as claimed in claim 1 wherein theilluminating light is laser light.
 4. The method as claimed in claim 1wherein the illuminating light is lamp light.
 5. The method as claimedin claim 1 wherein the illuminating light is polarized light.
 6. Themethod as claimed in claim 5 wherein the illuminating light is linearlypolarized light and wherein said pair of complementary polarizationcomponents are parallel and perpendicular, respectively, to saidlinearly polarized light.
 7. The method as claimed in claim 5 whereinthe illuminating light is circularly polarized light and wherein saidpair of complementary polarization components are right-hand andleft-hand circularity.
 8. The method as claimed in claim 5 wherein saidilluminating step comprises emitting light from a light source andpassing said light through polarizing means.
 9. The method as claimed inclaim 1 wherein said detecting step comprises passing the backscatteredlight through analyzer means and measuring the intensity of the analyzedbackscattered light with a photodetector.
 10. The method as claimed inclaim 9 wherein said photodetector is selected from the group consistingof a photomultiplier, a photodiode and a CCD camera.
 11. The method asclaimed in claim 1 wherein said image forming step is performedpoint-by-point or over an area using a photomultiplier or a photodiodeor a CCD camera or an equivalent photodetector.
 12. The method asclaimed in claim 1 wherein, prior to said image forming step, thebackscattered light is passed through a spatial gate such as 4F Fourierspatial gate.
 13. The method as claimed in claim 1 wherein said formingstep comprises calculating one of a ratio, a difference and acombination of a ratio and a difference of the pair of complementarypolarization components of the backscattered light and using said ratio,difference or combination to form an image.
 14. The method as claimed inclaim 13 wherein one of said pair of complementary polarizationcomponents is I_(a), the other of said pair of complementarypolarization components is I_(b) and said ratio, difference orcombination thereof is selected from the group consisting of I_(a)-I_(b), I_(a) /I_(b), I_(a) -I_(b) !/ I_(a) +I_(b) !, I_(b) !/ I_(a)-I_(b) !, I_(a) -I_(b) !/ I_(b) ! and I_(a) -I_(b) !/ I_(a) !.
 15. Themethod as claimed in claim 1 wherein the turbid medium is a tissuesample.
 16. The method as claimed in claim 15 wherein the tissue sampleis a human tissue sample.
 17. The method as claimed in claim 1 whereinthe illuminating light is a pulse of linearly polarized light, whereinsaid pair of complementary polarization components are parallel andperpendicular, respectively, to said linearly polarized light andwherein said forming step comprises subtracting the perpendicularpolarization component from the parallel polarization component to yielda difference and using said difference to form said image.
 18. A methodfor imaging an object located in or behind a turbid medium, said methodcomprising the steps of:(a) illuminating an object in or behind a turbidmedium with light, whereby light is backscattered from the object in orbehind the turbid medium; (b) detecting a pair of complementarypolarization components of the backscattered light; and (c) forming animage of the object using the pair of complementary polarizationcomponents.
 19. The method as claimed in claim 18 wherein theilluminating light comes from a pulsed or continuous light source (lamp,laser).
 20. The method as claimed in claim 18 wherein the illuminatinglight is unpolarized.
 21. The method as claimed in claim 18 wherein theilluminating light is linearly polarized light and wherein said pair ofcomplementary polarization components are parallel and perpendicular,respectively, to said linearly polarized light.
 22. The method asclaimed in claim 18 wherein the illuminating light is circularlypolarized light and wherein said pair of complementary polarizationcomponents are right-hand and left-hand circularity.
 23. The method asclaimed in claim 18 wherein said image forming step is performedpoint-by-point or over an area using a photomultiplier or a photodiodeor a CCD camera or an equivalent photodetector.
 24. The method asclaimed in claim 18 wherein, prior to said image forming step, thebackscattered light is passed through a spatial gate such as 4F Fourierspatial gate.
 25. The method as claimed in claim 18 wherein said formingstep comprises calculating one of a ratio, a difference and acombination of a ratio and a difference of the pair of complementarypolarization components of the backscattered light and using said ratio,difference or combination to form an image.
 26. The method as claimed inclaim 25 wherein one of said pair of complementary polarizationcomponents is I_(a), the other of said pair of complementarypolarization components is I_(b) and said ratio, difference orcombination thereof is selected from the group consisting of I_(a)-I_(b), I_(a) /I_(b), I_(a) +I_(b) !/ I_(a) -I_(b) !, I_(b) !/ I_(a)-I_(b) !, I_(a) -I_(b) !/ I_(b) ! and I_(a) -I_(b) !/ I_(a) !.
 27. Themethod as claimed in claim 18 wherein the turbid medium is a tissuesample.
 28. The method as claimed in claim 18 wherein the illuminatinglight is a pulse of linearly polarized light, wherein said pair ofcomplementary polarization components are parallel and perpendicular,respectively, to said linearly polarized light and wherein said formingstep comprises subtracting the perpendicular polarization component fromthe parallel polarization component to yield a difference and using saiddifference to form said image.
 29. A method of forming an image of aturbid medium, said image being free of surface image information, saidmethod comprising the steps of:(a) illuminating said turbid medium withlinearly polarized light, whereby light is backscattered from the turbidmedium; (b) detecting the parallel and perpendicular polarizationcomponents of the backscattered light; and (c) forming an image of theturbid medium using the perpendicular polarization component.
 30. Amethod of forming an image of a turbid medium, said image being free ofspecular reflection, said method comprising the steps of:(a)illuminating said turbid medium with linearly polarized light, wherebylight is backscattered from the turbid medium; (b) detecting theparallel and perpendicular polarization components of the backscatteredlight; and (c) forming an image of the turbid medium using theperpendicular polarization component.